It is known to use polyisocyanates whose reactive groups are closed by reaction with monofunctional combinations to produce mixtures of polyisocyanates and polyoxy combinations that can be stored at normal temperature. Such products are usually called capped (or blocked) polyisocyanates and are described, for example, in the Annals, Vol. 562, 1949, pp. 205-229. Under the effects of increased temperature these products split back into their original materials allowing the liberated isocyanate groups in turn to react with the polyoxy combinations present in the reaction mixture.
An example of such a single component enamel system that can be stabily stored at room temperature is that described in the German OS No. 23 46 818. It comprises an adduct of 3-isocyanatomethyl-3.5.5-trimethylcyclohexylisocyanate (isophorondiisocyanate=IPDI) dissolved in an enamel solvent with a high boiling point and a polyester containing hydroxyl groups based predominantly on aromatic dicarboxylic acids and a mixture of di- and tri-valent alcohols. This enamel system hardens at 180.degree.-350.degree. C. However, this system's hardening is too slow for some applications. Yet it is possible to considerably increase the hardening speeds by the use of catalysts such as tertiary amines or organotin compounds, for example. However, undesired secondary reactions often occur, especially in the presence of combinations that are capable of reacting with isocyanates. For example, they even split off monofunctional combinations from capped polyisocyanates at normal temperatures, making the mixture unstable.
It has now been found, surprisingly, that the hardening time can be shortened and the hardening temperature lowered by using hardeners comprising .epsilon.-caprolactam capped diisocyanates and/or .epsilon.-caprolactam capped polyisocyanates containing isocyanurate groups. The result is that hardening is quicker and can take place at lower temperatures the greater in proportion to increasing concentrations of isocyanurate in such mixtures.
The object of the present invention is a single component enamel, containing, when dissolved in a high boiling point, inert solvent,
(a) a polyester containing more than one hydroxy group in the molecule, and
(b) a mixture, as the hardener component, of .epsilon.-caprolactam blocked isocyanurates of the following formula: ##STR1## x is an integer of 1 to 4 or 0, and when zero, said formula is reduced to: ##STR2##
In formulas II and I R is --(CH.sub.2).sub.n -- wherein 3.ltoreq.n.ltoreq.12 or ##STR3## The hardener of formula II is itself a mixture of single compounds wherein the value of x is 1, 2, 3 or 4. The average value x of this mixture depends on the trimerization conditions. In the usual production methods the components with higher numerical values for x will occur less frequently.
The proportion of hardeners I and II can notably be varied within broad limits. For example, excellent results are achieved by mixing in, per OH-equivalent of the polyhydroxyl combinations, an equivalent capped NCO of the hardener mixture consisting of 99 to 0 parts by weight of hardener I and 1 to 100 parts by weight of hardener II, preferably 70 to 30 parts by weight of hardener I and 30 to 70 parts by weight of hardener II.
It is frequently advantageous to leave an amount of free NCO-, i.e., uncapped NCO groups, in the isocyanate adducts capped with .epsilon.-caprolactam in an amount of 0.1 to 10% by weight NCO, preferably 1 to 7% by weight NCO.
The hardener mixtures can be produced according to the following methods: